CHES2015 Writeup

CHES 2015

Ongoing! Writeups will be published only on 13/09/2015, obviously!

On https://ches15challenge.com/ four challenges were made available on 24/06/2015 21:00 CET and the CTF was open till 13/09/2015 8:00 CET, first day of the CHES2015 conference.
A backup is available here but the third challenge is online and needs the corresponding server to be up and running.
The Challenge is composed of 4 sub-challenges, which can be solved in any order. Each comes with at least one ciphertext, which contains a secret flag. Except if stated otherwise by the challenge itself, the ciphertext has been obtained thanks to an AES128 encryption in CBC mode with a null initialization vector. An intermediate flag can also appear during the resolution.

Challenge 1

slowed.wav
ctxt1.txt

Challenge 2

Cseh.jpg
ctxt2.txt

Challenge 3

client.c
ctxt3a.txt
ctxt3b

Challenge 4

rom.png
ctxt4.txt

file rom.png
rom.png: PNG image data, 30598 x 10240, 8-bit/color RGB, non-interlaced

That's a huge image!
To give an idea, here is a scaled-down version:

convert rom.png -resize 10% rom_scaled.png

And a zoom at scale 1:1 on the top left corner:

convert rom.png -crop 300x300-0-0 +repage rom_corner.png